Initial thoughts:
1. Even assuming both person A and person B use the service within a week of each other, the odds that they both use the same hastag are astronomically low. Person A says '#wannaGoOut', but person B says '#IWantToGoOut'. There are way too many possible permutations of a single thought.
2. As another commenter said, the design is clean but feels like it belongs in 1995. It will not get off the ground looking like this. The basic layout is fine, but the colors and fonts need to go.
3. Without an app, I think you'll have a hard time gaining critical mass. If someone has a thought and all it takes to get it out onto Thoughter is to (i) open the app, (ii) type, (iii) and send, then it will be way more likely to be used.
4. This feels a bit like Craigslist Missed Connections. Not saying that's a good - or bad - thing, it just does.
5. What if I see a cute girl at a coffee store, we hit it off, but I don't have her email address - or even her name? What then?